In a diesel engine, fuel injected into intake air is self-ignited due to compression of the air, however when the engine is start up cold, the temperature of the intake air may not rise to the point of self-ignition of the fuel. To deal with this problem, a glow plug may be installed in the engine combustion chamber and the air heated directly, or part of the exhaust gas may be recycled into the intake system so as to raise the temperature of the intake air.
Tokkai Sho 59-147831 published by the Japanese Patent Office in 1984 discloses a startup controller wherein fuel injection is not performed for a certain period after engine cranking has started. This is because when fuel is injected when the combustion chamber has not reached the self-ignition point, the injected fuel lowers the chamber temperature, and unburnt gas or incompletely burnt gas is ejected into the atmosphere.
In order to accelerate engine startup, an essential prerequisite is to suitably control the fuel injection timing. The temperature of the combustion chamber increases after the piston has repeatedly compressed the intake air, the temperature at top dead center first reaching a sufficiently high level for ignition to occur. To accelerate engine startup, therefore, the fuel injection timing must be controlled so that ignition occurs in the vicinity of top dead center, and for this purpose, the injection timing must be advanced considering the delay that exists between injection and ignition, i.e. the ignition delay period.
In a mechanical fuel injection pump, this fine setting of the injection period is difficult, because the injection timing follows contraction and expansion of wax according to the cooling water temperature in this type of pump.
In a fuel injection pump wherein the injection timing follows the oil pressure via a timer piston, the timer piston is generally biased toward the retardation side by a spring and advanced by the off pressure against the force of the spring. In this case, the oil pressure rises in proportion to the pump revolution speed, but it is difficult to obtain sufficient pressure when the engine is cold, because the cranking speed of the engine is also low when it is cold. Further, as the fuel injection amount is increased during startup, the reaction due to driving the pump increases. Hence, even in an electronic fuel injection pump, it is difficult to freely vary the injection timing during cold startup.
During cold startup, therefore, some time is required for the desired injection advance angle to be obtained. However, if the fuel injection timing is not appropriate, a certain time will be required from the start of fuel injection to good ignition and a full combustion even when fuel injection is not performed for a certain time after cranking begins, as described in the aforesaid prior art example. During this time, the temperature of the combustion chamber falls due to vaporization of the injected fuel, and that makes it increasingly difficult to shorten startup time. In addition, emissions of toxic components of incomplete combustion increase.